The timing of the initial collision between the South and North China blocks constraining from the sediments in the eastern Sichuan Basin

In this study, detrital zircon U–Pb geochronology, trace element and Hf isotopic compositional data from the Early-Middle Triassic clastic rocks in the eastern Sichuan Basin were obtained to distinguish the sediment provenance and constrain the timing of the initial collision between the South China and North China blocks. Detrital zircons from the Early Triassic Feixianguan Formation clastic rocks yield one major age peak at 2476 Ma and three minor age peaks at 1886, 802 and 304 Ma. These detrital zircons may be derived from the South China Block. Detrital zircons from the Early Triassic Jialingjiang Formation clastic rocks yield multiple age peaks at 979, 856, 392 and 269 Ma, indicating a mixed sediment provenance from the South China Block and Qinling Orogenic Belt. This is the first appearance of the detritus with the Qinling Orogenic Belt affinity in the eastern Sichuan Basin. Detrital zircons from the Middle Triassic Leikoupo Formation clastic rocks yield two centralized age peaks at 447 and ca. 245 Ma. These zircons may mainly be derived from the Qinling Orogenic Belt. The results indicate an abrupt change in the detrital zircon U–Pb provenance from the South China Block to the Qinling Orogenic Belt during the Early-Middle Triassic. Integrating the provenance change and other geological characteristics, we suggest that the initial collision in the eastern Qinling Orogenic Belt occurred in the Early Triassic.


Qinling Orogenic Belt
The Qinling Orogenic Belt is situated between the South China Block in the south and the Nouth China Block in the north (Fig. 1), and stretches thousands of kilometres.It is a crucial geologic and geographic boundary in the China Continent and even the Eastern Asia Continent 20,21 .The Qinling Orogenic Belt is a large-scale compound orogenic belt that has experienced multistage oceanic subduction, continental collision and intracontinental orogeny 22,23 , and has been considered to have formed by the eventual collision between the South China and North China blocks 9,24 .This belt can be segmented into four tectonic units: the northern margin of the South China Block, South Qinling Belt, North Qinling Belt and the southern margin of the North China Block from south to north 23 .Moreover, the Qinling Orogenic Belt can be divided into the eastern Qinling Belt and western Qinling Belt by the Chengdu-Baoji railway 25 .
The southern margin of the North China Block is located between the Lingbao-Lushan-Wuyang fault and Luonan-Luanchuan fault, and has a structural affinity to the North China Block 26 .The North Qinling Belt is located between the Luonan-Luanchuan-Fangcheng fault and the Shangdan suture zone.It consists of the Precambrian basement, the Neoproterozoic and Early Palaeozoic ophiolites and volcanic-sedimentary sequences, which are unconformably overlain by the Carboniferous-Permian sedimentary successions 24 .The South Qinling Belt is located between the Shangdan suture zone and the Mianlue suture zone, and is characterised by a south-vergent imbricated thrust-fold 23 .It mostly consists of the Neoarchean basement, the Neoproterozoic volcano-sedimentary sequences, and the Ediacaran to Middle Triassic sedimentary successions 9 .It separated www.nature.com/scientificreports/from the South China Block under the influence of the Mianlue Ocean during the Devonian to Early Triassic, and bordered the northern margin of the South China Block by continent-continent collision in the early Mesozoic.The northern margin of the South China is separated from the Qinling Orogenic Belt by the Mianlue-Bashan-Xiangguang fault 9 .

Study region and sampled horizons
The study sections are situated in the eastern Sichuan Basin and lie very close to the eastern Qinling Orogenic Belt (Fig. 1a).The continuous Triassic stratigraphic succession in the study region is mainly marine carbonate and well exposed, which includes the Early Triassic Feixianguan and Jialingjiang, Middle Triassic Leikoupo Formation, and Late Triassic Xujiahe Formation from oldest to youngest 27 .The Feixianguan Formation is mostly composed of limestone, dolomite, mudstone and siltstone, and is in conformable contact with the overlying Early Triassic Jialingjiang Formation.The Jialingjiang Formation mostly consists of carbonate-evaporite successions dominated by limestone, dolomite, and gyprock.The Leikoupo Formation is the youngest marine carbonate in the basin, which marks the final cessation of marine sedimentation 28 .This formation mainly consists of limestone and dolomite with interbedded evaporite and shale, and is marked by the so-called "mung-bean-rock" at the bottom 29 .The late Middle Indosinian movement generated the uplift of the study region, and the upper part of the Leikoupo Formation underwent denudation in varying degrees 27 .The Leikoupo Formation conformably overlies the underlying Jialingjiang Formation, and an unconformity underlies the overlying Xujiahe Formation.

Analytical methods
All analyses were conducted at the Institute of Mineral Resources, Chinese Academy of Geological Sciences.U-Pb isotope and trace element analysis for were conducted using Finnigan Neptune inductively coupled plasma-mass spectrometry (ICP-MS).A spot size of 25 μm and a 10-Hz repetition rate at 2.5 J/cm 2 were used for all analyses.Zircon 91500 was used as an external standard.The analytical procedures were the same as described by previously 33 .Age calculations and concordia diagrams were made using ISOPLOT.Zircon ages older than 1000 Ma were taken as 207Pb/206Pb ages, whereas the ages younger than 1000 Ma were taken as 206Pb/238U ages 34 .Zircon Lu-Hf isotopic analyses were conducted using a Neptune Plus multiple collector inductively coupled plasma mass spectrometer (MC-ICP-MS), with two different spot sizes of 32 and 60 μm.The GJ-1 was used as a reference material, and helium was used as the carrier gas.Operating circumstances and procedures were as described by previously 35 .

Result Detrital zircon U-Pb ages
A total of 420 zircon grains were analysed for U-Pb isotopes, 336 zircon grains with concordance exceeding 90% are used in the discussion, and the results are presented in Supplemental Table 1.The zircon grains of the four samples were mostly euhedral-subhedral and prismatic.The length of grains generally varies from 50 to

Detrital zircon trace elements
The detrital zircon trace element compositions are presented in Supplement Table 2.The Th/U ratios (moslyt > 0.1) of the detrital zircons from the samples (Fig. 6) reveal a magmatic origin.Most zircon grains show enrichment in LREE and flat HREE pattern with positive Ce anomalies and negative Eu anomalies (Fig. 6), which is consistent with a magmatic origin 37 .Zircons display dominantly continental signatures (Fig. 7a,b), and mainly are arcrelated/orogenic origins (Fig. 7c,d).Most grains plot the field of the felsic rock (Fig. 7e,f).
Detrital zircons with ages of 2700-2400 Ma from sample QJP-39-R-    China 58 and mostly occur on the western and southeastern margin of the Yangtze Block 44 .The εHf(t) values of the zircons from the Neoproterozoic magmatic rocks along the margins of the Yangtze Block are similar to those from the detrital zircons with ages of 900-700 Ma 59 .However, the rocks in this age population are rarely exposed in the North China Block 43,60 .Detrital zircons in the range of 340-280 Ma are a minor component, with a major age peak of 306 Ma.The coeval rock units are only distributed in the southeastern South China Block, such as: gneissic granite with a U-Pb age of 313 ± 4 Ma in northeastern Fujian Province 61 .Meanwhile, detrital zircons of this age have been widely reported in the underlying Late Permian clastic sediments in the Fujian Province 62 .Therefore, these zircons from sample QJP-39-R-1 are suggested to be mainly derived from the South China Block.

Provenance of the Jialingjiang Formation
The detrital zircons in the range of 1100-800 Ma constitute a dominant age group, with major age peaks at 979 Ma and 856 Ma and mostly negative εHf(t) values of − 20.1 to 9.8.These ages are corresponding to the Neoproterozoic and Grenvillian ages related to the amalgamation and break-up, respectively, of the Rodinia supercontinent 57,63 .Detrital zircons in the range of 1100-800 Ma are euhedral and angular, indicates a short-distance deposition.Thus, these zircons likely derived from the adjacent terranes.The Neoproterozoic magmatic rocks are widely distributed along the margin of the Yangtze Block, North Qinling Belt and South Qinling Belt.Grenville-age magmatic rocks are widely distributed in the North Qinling Belt with ages of 979-911 Ma  68,69 .The detrital zircons in the range of 2800-2400 Ma account for a small portion of the total zircons, and mainly derived from the Neoarchean to early Palaeoproterozoic crystalline basement of the Yangtze Block.The detrital zircons in the range of 300-250 Ma and 1800-1600 Ma are minor components, and derived from the Qinling Orogenic Belt 70,71 and the western margin of the Yangtze Block 72 .In addition, the Jialingjiang Formation in the Zigui Basin has a large number of ca.247 Ma detrital zircons and were deemed to derive from the Qinling Orogenic Belt, and the Precambrian detrital zircons might derive from the South China Block 12 .Hence, the results indicate a mixed sediment source for the Jialingjiang Formation sample that the Qinling Orogenic Belt and South China Block were the major source areas.

Provenance of the Leikoupo Formation
The detrital zircon age spectrum of the Leikoupo Formation samples differs from those of the underlying Lower Triassic samples (Fig. 6).Our samples and those of the Badong Formation in the Zigui Basin 12 exhibit similar features with major populations ranging from 2600-2400 Ma, 1950-1700 Ma, 1000-900 Ma, 500-400 Ma and 300-245 Ma.The detrital zircons in the range of 500-400 Ma constitute a dominant age group of sample JF-43-R-1, with an age peak of 447 Ma and negative εHf(t) values.The early Palaeozoic (450-420 Ma) magmatic activity occurred mainly in the North Qinling Belt, which was correlated with the northward subduction of the Shangdan Ocean 24,67 .Thus, the contemporaneous granitoids were well-preserved in the North Qinling Belt, and have negative εHf(t) values 21,68,73 17,74 .Hence, the Precambrian detrital zircons are a small composition of the total zircons, and yield three www.nature.com/scientificreports/major age peaks at 2505, 1851 and 977 Ma.The Jiangnan old land underwent tectonic uplift in the South China Block during the Middle Triassic, which resisted the transport of clastic materials from the Cathaysia to Yangtze blocks 75 .In addition, the Neoproterozoic magmatic rocks also sporadicly distributed in the Qiangtang Block 76 .The Precambrian detrital zircons may derive from the Qinling Orogenic Belt, Qiangtang Block and Yangtze Block.In conclusion, the main source region of the zircons in the Leikoupo Formation samples might have been the Qinling Orogenic Belt, and the South China Block was a minor source region.

Timing of the initial collision between the South China and North China blocks
Due to the plate collisions do not generate direct geologic records, it is necessary to constrain the timing of the initial collision using the pre-or postcollision geological events 16,77 .The collision between the South China and North China blocks contributed to the formation of the Dabie-Hong'an-Tongbai-Qinling Orogenic Belt, which stretches more than 2,000 kilometres 78 .The earliest contact of this collision occurred in the Dabie Orogenic Belt in the eastern part of the whole collision belt, and the timing of the collision yield a younging trend from east to west 14,79 .The high-pressure eclogite with an age of 252 Ma in the Dabie Orogenic Belt records the subduction of oceanic crust before continental collision 80 .Palaeontological and palaeogeographical data indicate that the water depth increased from south to north at the northern margin of the Yangtze Block, which faced the Mianlue Ocean at the end of the Permian 81,82 .Palaeomagnetic studies have proposed that the South China and North China blocks collided in the Late Permian-Early Triassic 5,83,84 .Therefore, the initial collision of the two blocks in the Dabie orogenic belt did not occur earlier than the last phase of the Late Permian.Furthermore, a series of geological evidences indicate that the final convergence of the South China and North China blocks in the western Qinling Orogenic Belt occurred posterior to the end of the Late Triassic.The western Qinling Orogenic Belt was in a deep-water basin environment during the Early Triassic to the early Late Triassic, and preserves thick marine sedimentary successions 85,86 .In addition, the Late (228-210 Ma) granites in syn-collision turning to post-collision were widly distributed in the western Qinling Orogenic Belt 24,87,88 .Finally, the Songpan-Ganzi marine residual basin is filled with a considerable thickness of Middle-Late Triassic flysch.These abundant continental sediments have been deemed to derive from the Qinling-Dabie Orogenic Belt 89,90 .Hence, a channel should exist in the interzone between the South China and North China blocks, which can transport much debris from the orogenic belt into the Songpan-Ganzi residual basin.Before the initial collision, the South China and North China blocks were separated by the Mianlue Ocean, and the Sichuan Basin only possibly received the source materials provided by terranes in the South China Block (Fig. 9a).After the collision, the Qinling Orogenic Belt was uplifted and underwent long-term eolation and denudation, and formed massive detritus.The Sichuan Basin, as a sedimentary region, is adjacent to the Qinling Orogenic Belt, and began to receive this detritus.The earliest timing of the detritus reaching the Qinling Orogenic Belt can precisely constrain the upper limit of the timing of the initial collision.The provenance analysis suggests that the continuous deposits of the Feixianguan, basins showing the first appearance of detritus from the Qinling Orogenic Belt happening in the Jialingjiang Formation 12 .The detritus from the Qinling Orogenic Belt in the Middle Triassic Leikoupo Formation is further increased and is the main source.Hence, provenance change suggests that the initial collision between the South China and North China blocks in the eastern Qinling Orogenic Belt occurred in the Early Triassic (Fig. 9b).The tectonic evolution characteristics and transition from ocean to land in the northern part of the Middle-Upper Yangtze Block further support the Early Triassic initial collision between the South China and North China blocks.The timing of the cessation of marine deposition can be used to constrain the upper limit for the collision timing 77 .The Middle Triassic Leikoupo Formation and contemporaneous strata in the northern part of the Middle-Upper Yangtze Block are a set of marine carbonate rocks that are widely distributed, and the overlying Upper Triassic is a set of typical terrestrial clastic deposits 81 .Therefore, the Middle Triassic Leikoupo Formation and contemporaneous strata can represent the youngest marine strata in the northern part of the Middle-Upper Yangtze Block during the early Mesozoic.However, the northern part of the Middle-Upper Yangtze Block, including the Sichuan Basin, is the shallow water sedimentary region in the passive continental margin.The cessation of marine sedimentation only represents the timing of the seawater retreat from the Middle-Upper Yangtze Block, and is not the timing of the closing of the eastern Qinling Ocean.Hence, the youngest marine strata in the northern part of the Middle-Upper Yangtze Block may be much later than the collision timing 1 .In conclusion, the youngest marine strata suggest that the initial collision between the South China and North China blocks in the eastern Qinling Orogenic Belt may have occurred earlier than the Middle Triassic.The Triassic succession in the Michangshan region in the northern Sichuan Basin developed a regional unconformity in east-west trending, which is parallel to the Mianlue suture zone.This is construed as tectonic records related to the point contact collision between the South China and North China blocks in the eastern Qinling Orogenic Belt 92,93 .The palaeontological characteristics in the South Qinling Belt also support the hypothesis that block collisions occurred in the Early Triassic.The similarity between bivalve species in the Zhen'an region in the South Qinling Belt and the South China Block is as high as 87% during the early Early Triassic.Meanwhile, the similarity is low between the Zhen'an region and the southern margin of the North China Block at the early Early Triassic, and this value significantly increased at the late Early Triassic 94 .The significant increase in similarity at the late Early Triassic may be related to the migration of bivalves from the South Qinling Belt to the southern margin of the North China Block, which indicates that collision between the South China and North China blocks had already occurred at the late Early Triassic.The different geological perspectives suggest that the initial collision between the South China and North China blocks in the eastern Qinling Orogenic Belt occurred in the Early Triassic.

Conclusions
Detrital zircon U-Pb geochronology, trace element and Hf isotope analyses were performed on the Early-Middle Triassic samples from the eastern Sichuan Basin, northwestern South China.Integrated with the previous research results, the following conclusions in this study could be obtained:

Figure 2 .
Figure 2. (a) Geological sketch map of the study region and sample locations 30 ; (b) Cross-sections in the study region.The Shangbaiyang section is modified from 31,32 .

Figure 3 .
Figure 3. Field photographs showing the sampling locations and the photomicrographs of the samples in the study regions.(a-c) Argillaceous siltstone at the bottom of the Feixianguan Formation; (d-f) Argillaceous dolomite at the top of the Jialingjiang Formation; (g-l) Shale at the Leikoupo Formation.

Figure 4 .Figure 5 .
Figure 4. Representative cathodoluminescence (CL) images of representative zircon grains from samples and their U-Pb ages and ε Hf(t) values.White and black circles represent the analytical sites of the zircon U-Pb ages and yellow circles represent the analytical sites of the zircon Hf isotopes.

Figure 6 .
Figure 6.Th/U ratios and chondrite-normalized REE patterns of the detrital zircons.Normalization values of chondrite and primitive mantle are from 36 .

Figure 8 .
Figure 8. Hf isotopic compositions of the detrital zircons from the samples.

Figure 9 .
Figure 9. Schematic diagram depicting the initial collision between the South China and North China blocks 91 .

( 1 )
The detrital zircons from the Feixianguan Formation may derive from the South China Block.The zircons from the Jialingjiang Formation have a mixed source from the South China Block and Qinling Orogenic Belt.The zircons from the Leikoupo Formation may mainly derive from the Qinling Orogenic Belt.(2)The abrupt provenance change in detrital zircons from the Early-Middle Triassic successions was most likely a result of the collision between the South China and North China blocks.Thus, we suggest that the initial collision between the South China and North China blocks in the eastern Qinling Orogenic Belt occurred in the Early Triassic.